The chassis of a vehicle substantially consists of front wheels (steering wheels) interconnected by a front axle, rear wheels (driving wheels) interconnected by a rear axle, and a frame supported on the axles and connecting the front and rear axles. A body is rigidly attached to the frame and in the case of most motor homes, the body is substantially wider than the frame and extends in width past the wheel width.
The frame and body are substantially non-flexible and will not accommodate twisting. Yet the vehicle is driven over road surfaces that are not level. Thus, whereas the axles are designed to be coplanar with the wheels resting on a level surface, the axles constantly change in angular relationship as the vehicle travels over the road surface. A suspension system interposed between the frame and axle is intended to accommodate the angular change of the axles.
One form of suspension system is an air suspension system. An air bag is pivotally placed at each of the four intersections whereat the two side rails of the rectangular frame cross over the two axles. Thus, the frame (and body) are supported on four air bags. Each bag is under air pressure (e.g., the front bags are under 45 PSI and the rear bags are under 65 PSI). The bags will expand when the frame is lifted away from the axles and they will compress when added weight is applied to compress the frame toward the axle. The bags are necessarily designed to restrict the amount of expansion and contraction and thus the permitted change in distance between the frame and axles at the positions of the bags.
As the vehicle travels down the road, a front wheel (and then the rear wheel) may encounter a dip (hole) or a rise (bump) in the road and in either event the front axle is skewed relative to the rear axle (or vice versa). Assuming a dip in the road surface that is engaged by the right front wheel, the right front wheel drops down and the front axle is angularly shifted downwardly left to right. Both the front bags (assuming they are inwardly of the wheels) drop down, the right bag more than the left. Thus, the front end of the frame and body drop down at least to the lowered position of the left bag. Dynamics dictate that the frame and body will then tilt toward the right front bag and this sets up an undesired rocking of the body. Nevertheless, the interaction of the bags allows the frame and body attached thereto to avoid a twisting force as would otherwise be urged upon the frame and body when one of the axles is angularly deflected relative to the other axle.
The above describes generally the function of the air bags but the reader should understand that other structural features of the vehicle affect the relative movement between the axles and frame. Shock absorbers dampen or resist the shifting of the frame relative to the axles and reduces the rocking effect that occurs as the vehicle travels over small rises and dips in the roadway. However, even the reduced rocking when applied to a motor home causes discomfort. The frame of a motor home is substantially more narrow than the wheel width and the body width of the motor home. Whatever deflection occurs at the inner position of the air bag, i.e., located under the frame rails, that deflection is magnified at the side edges of the body which overhangs the bag position and a slight rocking effect at the bag position turns into a more severe rocking or tipping effect experienced by the passengers of a motor home.
Furthermore, it is believed that the overhang produces a lever effect and the force differential as applied to the bags is increased. Thus, what might be a small deflection as between the axles produces a more severe force differential as applied to the air bags and that differential produces greater rocking than necessary to accommodate the axle deflection.
In addition to the added width, a motor home is tall. The floor of the motor home is built over the chassis and the ceiling height must accommodate a six-foot plus person. The total height, road to roof, will likely exceed ten feet. The center of gravity is thus higher than in other vehicles and centrifugal force when rounding curves will urge tipping. The air bag at the outside position is a pivot about which the centrifugal force is directed. Tipping or tilting is increased as the pivot is moved inwardly under the center of gravity and with the bags located under the frame, the typical motor home incurs considerable and undesirable tipping when rounding a curve. High winds directed crossway to the direction of travel create a similar problem.
It is known that one motor home manufacturer attempts to reduce the undesired rocking by simply positioning the bags at the extreme side edges of the body. The frame and body are made extra strong and are structurally integral one with the other. The bags allow a certain degree of angular deflection of the axles relative to the frame and body but rocking or tipping is substantially reduced. Whereas this modification reduces the rocking motion, the vehicle offers a smooth ride on relatively level roadways, e.g., a freeway. A problem occurs, however, when maneuvering the vehicle over more uneven surfaces, e.g., through parking lots, over traffic bumpers and curbs and the like. The bags only allow limited angular deflection of the axles and when the roadway surface variation demands more than what is permitted (as often it will), a severe twisting force is exerted on the frame and body. Twisting of the frame and body is avoided only if the frame is sufficiently strong to withstand the cantilever effect of shifting all of the weight to three of the wheels, e.g., with the fourth wheel literally lifted off the roadway. The stress applied to the body and frame are great and to protect the body and frame from twisting, the structure has to be made substantially stronger resulting in added weight and added cost of construction.